The present invention relates to a stationary radio station according to the preamble of appended claim 1.
The present invention relates more particularly to a radio station which constitutes a base-station for a wireless telecommunications system for telecommunication to/from mobile radio units which are within the station's area of coverage.
With wireless telecommunication, the strength of the signal can vary to a large degree between different positions of a receiver antenna. This very local variation is due to fading caused by multi-way propagation, which in turn is due to reflections in the environment.
It is known to solve this problem by so-called diversity reception. Today there are mainly two different types of diversity reception and it is known to use, in one and the same system, either so-called space-diversity or so-called polarisation-diversity. Space-diversity means that two or more receiver antennas are placed at a suitable distance from each other so that the different antennas are not effected by fading, due to multi-way propagation, at the same time; see for example EP 0 517 196 A2. Polarisation-diversity means that radio signals are received both with horizontal and vertical polarisation, either in one and the same antenna with two feeders, one for each polarisation, or with two antennas, whereby due to different polarisation, fading does not occur at the same time in respective feeders or antennas.
The antennas are coupled to a common unit which presents means for selecting the strongest signal at that time, or for combining signals in order to obtain the best possible signal-noise ratio. There are a plurality of well-known algorithms for selecting/combining the signals, e.g. MRC.
The diversity gain i.e. the sensitivity of the receiver, increases with the number of independent channels, for which reason it is desirable to lay out a large number of antennas. A disadvantage with the solutions known up until now is that they require a great deal of space and are very costly. With space-diversity, the antennas namely need to be placed a certain minimum number of wavelengths from each other. Even with polarisation-diversity, an increased space requirement exists. Since the units are often installed in places which are positioned high-up and are thereby difficult to access, the installation cost is a very large part of the total system cost and is in proportion to the number of necessary units.
At the same time as large problems can arise in highly built-up areas due to multi-way propagation, there are economic as well as aesthetic reasons giving rise to a desire to minimize the number of antennas and radio units.